As a multiple-input multiple-output (MIMO) technology can effectively improve spectral efficiency and a transmission rate, the technology has been widely applied to modern communications systems, for example, a multiuser wireless communications system. A base station (BS) in a multiuser MIMO downlink system communicates with multiple users by using a same time-frequency resource, and a user inevitably suffers interference from multiple users. To reduce such interference, beamforming is generally implemented at a base station end, so that a signal is transmitted directionally or received directionally.
According to an existing protocol, both a base station and a terminal store a set of precoding codebook, and the precoding codebook includes multiple precoding matrices that are used to indicate a beam direction. The precoding matrices are relatively complex; therefore, for ease of retrieval and transmission, each precoding matrix is set to be corresponding to one code word.
In a communication process, the base station sends pilot information to a user. After receiving a pilot signal, the user performs channel estimation according to the pilot signal, and selects, according to a channel estimation result, an optimal code word from the precoding codebook stored in the terminal. After selecting the optimal code word, the terminal encapsulates the optimal code word in a PMI (Precoding Matrix Indicator) message and sends the PMI message to the base station. After receiving the PMI message, the base station decapsulates the PMI message to obtain the optimal code word, and finds, according to the optimal code word, a corresponding precoding matrix from the precoding codebook stored in the base station. After finding the corresponding precoding matrix, the base station performs beamforming according to the precoding matrix that is found.
However, because a beam direction is continuous and unlimited and a precoding matrix is discrete and limited, a quantization error inevitably exists when the precoding matrix is used to indicate the beam direction. As a result, a beam cannot correctly point to the terminal. For example, as shown in FIG. 1, in an ideal state, directions of beams between a base station 110 and two terminals 120 should be shown by dashed lines in the figure, and correctly point to the terminals 120. However, as shown by solid lines in the figure, directions of beams between the base station 110 and the two terminals 120 deviate from correct directions due to a quantization error and cannot correctly point to the terminals 120.